This invention relates generally to the assembly of structures, and in particular to a press for manufacturing trusses which provides several advantageous features.
Pre-manufactured structural frameworks, such as trusses, are widely used in the construction industry for forming a roof, wall panel, floor, or other building component. The truss is assembled to the correct specifications at a factory and then shipped to a construction site. Each truss includes a collection of structural members made of wood, plastic, or metal which are held together by fasteners, such as nailing or connector plates. Efficient assembly of the truss is facilitated by a press apparatus which drives the connector plates into assembled precut structural members where they intersect or abut each other.
In one widely used type of system, a press is suspended from an overhead carriage for movement between several splice pedestals (or stands) supporting the structural members in assembled position. Each of the pedestals includes a holder for holding a lower connector plate at a position below the structural members and bridging lower surfaces of the structural members at their intersection or abutment. An upper connector plate is placed over the joint so that it bridges upper surfaces of the structural members. The press has a C-shaped frame which carries upper and lower platens adapted to be positioned above and below the respective upper and lower connector plates. Actuation of a hydraulic powered cylinder causes the upper platen to move downwardly toward the lower platen and press the joint so that the connector plates are driven into the structural members thereby connecting the structural members.
There has been growing demand for larger, heavier trusses using larger sizes of connector plates, such as 8×8 inches and 10×12 inches, which require a larger capacity press, e.g., on the order of about 37.5 to 50 tons instead of 25 tons. Unfortunately, existing presses have a number of drawbacks which degrade their effectiveness in applying such a large force without substantial increases in size and weight of the frame.
Frames of the prior art are prone to fatigue damage. Typically, a frame has two major structural parts including an inner peripheral rim defining the inside edge of the C-shape and an outer peripheral rim defining the outer edge. For lower cost manufacturing (e.g., by forging of steel), the frame has a profile which is not a substantially rounded “C”, but rather a generally rectangular “C”. Consequently, the frame has two substantially 90° turns at corners of the C-shape, separating the generally horizontal and vertical portions of the “C”. During operation, the frame is exposed to a reaction force urging apart the upper and lower platens. Unfortunately, stress concentrations arise at each turn which produce a local stress greater than a nominal stress. Consequently, the frame tends to develop fatigue cracks and fail sooner than should be expected for its size and loading. Aggravating this problem is that the majority of the load is transmitted through the inner peripheral rim, which consequently exhibits the earliest fatigue damage. The inner and outer rims are divided such that the loads carried by each are separate, without the added stability or efficiency if the load was shared in a structural framework.
Systems of the prior art are not designed for rapid maintenance and repair. The hydraulic cylinder for driving the upper platen includes a tubular body holding a reciprocally movable piston connected to a movable rod. That body is typically welded to the frame. Consequently, the body carries load and is subject to fatigue damage, particularly along the weld. Replacement of the cylinder is difficult and requires substantial down time. Moreover, maintenance work on the cylinder or its replacement with a new or differently sized cylinder and piston is a major repair effort. There is no flexibility in quickly substituting differently sized cylinders for carrying different loads tailored to the truss. The cylinder and its tubular body are not “off the shelf” items.
The upper platen is subject to failure when used with high loadings. Periodically, the platen inadvertently presses a non-flat object, such as due to operator error or due to an incorrectly positioned stop on the pedestal. That exposes a portion of the platen to an even greater load which frequently leads to permanent deflection or failure.
Operationally, presses of the prior art are inefficient. An operator controls a switch to activate the hydraulic cylinder and apply force through the cylinder to the joint. The operator makes a visual judgment of whether the connector plates are completely embedded into the structural members, and releases the switch so that the platens may separate. Often, the operator misjudges that time and must conduct one or more repetitive cycles of force application. Further, the press may be limited in magnitude of force due to the aforementioned structural drawbacks and cylinder size and requires several cycles to embed larger connector plates. Thus, substantial delays may occur in the construction of a roof truss.